In this day of expanded use of video display terminals in almost every conceivable workplace, there are vast numbers of VDT operators who have reported, through the results of several field surveys, an eye strain and fatigue that leads to ocular discomfort and transient visual impairment. These physiological effects, which may have been initially overlooked, have now been found to lead to time loss, inefficiencies and extended rest periods that decrease the significant advantages attained through the use of the expensive VDTs. While some observers, concerned over these problems, initially thought the origin of the discomfort was solely due to the radiation, intensity or contrast, displayed on the screen of the VDT, it has been found that the image produced on the VDT is not the sole, if even the significant, culprit.
When the visual display terminal was first brought to the workplace its functions and capabilities were heralded to the extent that little thought was given to the environment of the VDT. For instance, in the majority of workplaces the VDT sits atop a desk or other workplace for which the lighting was designed for work at the traditional desk top. This illumination in offices and other workplaces is derived from the conventional light sources, windows and reflections from a variety of objects and surfaces that also includes the operator. The positioning of the VDT in the workplace under these conditions did not take into consideration that the VDTs differ from most other work objects or surfaces in that they have luminance and emit light as well. All VDTs usually have a highly specular curved transparent surface positioned in a more or less vertical plane. Reflections from this mirror-like front surface of the screen form images as do reflections from the luminance of windows and other objects. These reflections can produce rays of light forming apparent images of far or near objects such as keyboards, desk tops, walls, clothing, reflection of windows or other luminaries to produce a distinct glare over portions of the screen resulting in a reduction in the contrast and visibility of the display characters.
Long experience makes it quite apparent that extraneous lighting and reflections may cause visual problems for video display terminal operators. The reflection of a lamp or a bright window or colorful wearing apparel upon any viewing screen renders it difficult or impossible to comfortably view the image on the screen. No one who has ever watched television or worked at a VDT, microfilm viewer or similar display device has not had similar visual difficulties.
Visual problems also arise due to the VDT being an illuminated light source. Operators may encounter difficulty in successfully viewing a VDT screen and other direct or indirect light sources having luminances much different from that of the screen. For instance, if an operator looks toward a window and then looks back towards the screen several visual difficulties may occur. First, discomfort may be caused by the large differences in luminance between the screen and the window, and second, the visibility of the display image may be reduced for several seconds as the visual system of the operator adapts from the high luminance of the window to the much lower luminance of the VDT screen. This is referred to as the transient adaptation effect and is particularly important when a positivecontrast display is used.
Though temporary visibility loss is due to transient adaptation in VDT operations, it occurs not only, when the operator looks alternately between the glare source and the display screen, but also will occur if a secondary lighting is used on a source document along with viewing the VDT screen. This transient adaptation is a temporary loss of visibility that occurs at any time when an operator changes his point of regard to surfaces having different luminances, or when the illumination changes occur naturally in the visual environment.
The loss is particularly noticeable when a positive contrast display is utilized along with a negative contrast source document, such as a typewritten page. A temporary reduced visibility of the display images can also be caused by a scattering of light within the eye which reduces contrast at the retina or by the specula reflections that produce reflected glare over the display image. VDT images, appear to the eye of the observer to be located somewhat behind the screen, rather than at its surface. The accommodative and convergence systems of the eye may fluctuate between reflected images on the surface of the screen and the display image, usually resulting in an intermittent or constant blur of the display characters. This accommodative response is known as the time required to shift eye fixation and focus between near and far objects, and thus, bears significantly on visual acuity particularly after several hours of VDT work that includes transient changes in these near points of accommodation and convergence.
It has also been found that differences in task characteristics in various VDT related work have an effect on the visual acuity of the operators. For example, operators who spend a large portion of their time viewing the screen may experience more difficulty with reflected images and screen glare than do data entry operators whose job is typically less screen intensive. Data entry operators, however, may experience more difficulty with luminance differences among the source document, screen and background.
Environment lighting conditions that project light radiation toward the surface of a VDT screen using a cathode ray tube, are found to be reflected from the inside phosphor surface in a diffused manner without imaging the radiation. This incident light from external sources may excite the phosphor increasing its luminance and noticeably reducing contrast making viewing more difficult.
All these reflected images, not only produce a reduction in contrast with the display images, but also have been found by many operators to produce an ocular discomfort and visual impairment. Ocular complaints of workers have often been discussed in terms of eye strain and visual fatigue. These terms, however, are vaguely defined and do not correspond to known physiological or clinical conditions though glare is recognized universally to be undesirable.
Glare, as it is used throughout this invention, is the sensation produced by luminances within the visual field that are sufficiently greater than the luminance to which the eyes are adapted to cause annoyance, discomfort or loss of visual performance and acuity. The magnitude of this sensation of glare depends on factors such as the size, position and luminance of the light source or reflected surface, the number of light sources and the luminance to which the eyes are adapted. Reflected glare is the result of specular reflections from polished or glossy surfaces or diffused reflections that produce a veil of light that reduces contrast. Disability glare may be caused by light scattered within the eye, thereby reducing contrast at the retina, or by reflected glare to reduce visual performance and temporary loss of visibility. Discomfort glare produces discomfort and it may, but does not necessarily, interfere with visual performance or visibility, just as disability glare may or may not be accompanied by discomfort.
A lighting environment that is properly designed and therefore comfortable for workers performing traditional desk top tasks may not be comfortable for workers involving VDTs. The design of general office lighting assumes a depressed line of sight toward the lower working surface such as a table or desk. To the contrary, a VDT screen is viewed along the line of sight that is at or near the horizontal. This higher line of sight required to view the screen brings luminaries such as ceiling, walls, windows, closer to the line of sight resulting in a higher glare index and a greater likelihood of discomfort glare.
It is also apparent that reflections from the front surface of a VDT screen reduce the contrast and thus the visibility of the display image. Such reflections can also act as additional visual targets which may promote fluctuations in accommodation and convergence. Positive contrast may tend to make the pupil larger thereby reducing visual acuity or contrast sensitivity, increasing the blur circle and permitting greater spherical aberration. Reflections of ambient lighting by a VDT screen can cause a significant loss of character contrast on the screen. It has become apparent that two distinctively different types of reflection may occur, first, the specular reflection from the front glass surface of the CRT, and second, a diffuse reflection from the scattering of incident light upon the phosphor surface of the CRT. To date there are no accepted standardized procedures to measure either of these disturbing problems.
It is these reflections that are derived from ambient or environmental conditions that are included in the term "visual noise" and are the source of the eye strain and visual fatigue of the operators.
In addition to these reflections that produce visual noise, viewers of visual display terminals using a CRT have visual difficulties due to "after images", which are also a form of, and are included within the meaning of the term visual noise. Through the study of the physiology of the eye, much is learned about the origin of these after images. It is known through the Young-Helmholtz theory of color vision that there is present in the retina of the eye three separate and distinctly different photoreceptors, each detecting and transmitting a different signal to the visual cortex to enable the eye to sense and the observer to visualize different colors.
This sensing is known to involve a chemical process in which the protein rhodopsin is selectively bleached in the presence of particular wavelengths of light defining, for example, red, blue and green wavelengths. The bleaching is one of the pigments of rhodopsin sensitive to that particular wavelength of incident light. While this bleaching is temporary, and the pigment is reformed inversely related to the intensity of the illumination of the sensitive color, the remaining pigments of the photoreceptors can produce an after image of a complementary color when the eye is directed to a brightly lighted surface.
At times this phenomenon of the eye can be enjoyed as a curious amusement, but for operators of a CRT in which the indicia is typically green, yellow or white, there is no joy in experiencing these after images as they may continually form upon the brightly lighted source document. On the contrary, these continually presented after images become annoying and lead to eye strain and fatigue.